Along with the development of the network, the optical fiber communication becomes more and more important. In the optical fiber communication, not only the loss during transmission thereof is low also the transmission capacity is high. When cooperating with a DWDM (Dense Wavelength Division Multiplexing) system, an optical fiber can simultaneously transmit sixteen, thirty-two, or even more wavelengths. Since the spacing of the wavelengths is so close together, the shift of the wavelength will cause a serious crosstalk. Because of this situation, a wavelength monitoring of a laser source becomes very important. Although wavelength monitoring systems have been applied in the commercial product, they can, usually only monitor single wavelength and are unable to monitor the mode-hopping of the laser.
In the DWDM network system, a tunable laser is a key component for using as, for example, a spare source and a fast wavelength-switching device etc. Therefore, the wavelength monitoring of a tunable laser has to cope with multiple output wavelengths and the required tuning speed simultaneously.
Fabry-Perot (FP) etalons have been commercially used for wavelength control in the tunable lasers, but the FP etalon cannot distinguish among different channels due to a periodic wavelength characteristic thereof and thus cannot monitor the mode-hopping and incomplete tuning problem in the laser. Furthermore, employing the tunable FP etalon or an array waveguide grating (AWG) as a monitoring module will be limited by a response speed or an expensive cost. Even though a thin film filter is employed for monitoring the channel, the waveband for monitoring the channel is unchangeable without moving the filter.
As described in U.S. Pat. No. 6,339,492 B1, a tunable optical filter having a phase-shifter and a rotator is disclosed. The tunable optical filter includes a first linear polarizer, a birefringent plate, a Faraday rotator, and a second linear polarizer, wherein an order of arrangement of the birefringent plate and the Faraday rotator is exchangeable. Another aspect thereof is to additionally include a variable phase-shifter therein for achieving a shift of the spectral transmittance. In this patent, it is considered an optical tunable filter, and it doesn't involve the function of optical isolation.
Another description in U.S. Pat. No. 6,421,131 B1, a birefringent interferometer system is described. The birefringent interferometer system uses liquid crystal cells to produce orthogonal-polarization optical path differences (OPD). Further, retarders are also incorporated to extend the range of OPD. This patent is mainly applied in an optical spectral measuring system, however, also doesn't act as an optical isolator.
Because of the drawbacks described above, the applicant keeps on carving unflaggingly to develop a “simultaneous optical isolation and channel monitoring system” through wholehearted experience and research.